The invention relates to a voltage source converter system, and more specifically to a plane parallel bus bar assembly that is designed to allow high switching frequencies of a direct voltage component in the system. The invention also relates to the insulation of conductor bars incorporated in the bus bar assembly, and to a voltage source converter incorporating the bus bar assembly.
As used herein, “high switching frequencies” relates to frequencies reaching the order of approximately 1,500 Hz.
In the course of providing high quality electrical power and stabilized grid voltage, Voltage Source Converters (VSC) have been developed to control the magnitude, phase and frequency of the three phase AC-voltage. The VSC provides a capacity both to generate and absorb reactive power, mitigating voltage fluctuations generated by variations in loads and thus restoring voltage and current balance in the grid.
The main task for the voltage source converter is to inject corrective currents into each AC-phase of the network. Load variations in the grid are continuously monitored and processed for generating control signals to a switching device. The switching device allows high frequency switching of either one of the three levels of voltage (+U, O, −U) on the direct current side of the converter, into each AC-phase. If the switching frequency is high enough and switching is performed correctly, an almost sinusoidal current may be achieved in the AC-phases.
Implementation of semiconductor switches such as the Insulated Gate Bipolar Transistor (IGBT) valve makes it possible to achieve the desired switching frequencies. On the direct current side of the converter, a DC-capacitor of little or minimum energy storage capacity will be sufficient, since the converter average active power transfer is zero. The typical arrangement of a three level converter is diagrammatically illustrated by the simplified circuit of FIG. 1 in the drawings.
A fast commutation of current in the converter valves (high switching frequency) requires however, that inductance is reduced and kept to a minimum on the direct current side. In order to achieve low inductance, the direct current side comprises conductor bars that are closely spaced in side by side relation. Considering the voltage levels applied between the bars, typically in the order of 10–20 kV or more, it is a problem to secure sufficient insulation in the narrow gaps that are formed between the conductor bars.
Conventionally, conductor bars may be provided an insulation by wrapping, e.g., layers of insulating material about the conductor. The method is suffering from a limited accessibility to the conductor bar when electrically connecting to auxiliary equipment. The method is also suffering from a problem to secure an insulation that is free of air pockets, where partial discharges gradually may cause a break down of the insulation.
Polymer materials are known to have dielectric properties. In “Electric Field Reduction Due to Charge Accumulation in a Dielectric-Covered Electrode System”, IEEE Transactions on Dielectrics and Electrical Insulation, Vol.7 No. 3, Jun. 2000, H. J. M. Blennow et al. discusses charge formation at the dielectric surfaces of an electrode system covered with silicone rubber.